Potent H.sub.1 receptor antagonists of various structural types are known, and are useful in treating the symptoms of inflammatory conditions such as allergic rhinitis, and allergic conditions of the skin, which are mediated at least in part by the release of histamine. However, in such conditions, in which histamine release plays a causative role, there may be other mechanisms at work which are not inhibited by treatment with an H.sub.1 receptor antagonist alone. For example PAF is released directly from cell membranes and mediates a range of potent and specific effects on target cells resulting in a wide variety of physiological responses, including hypotension, thrombocytopenia, bronchoconstriction, circulatory shock, increased vascular permeability (oedema/erythema), and accumulation of inflammatory cells in the lower airways.
There is therefore a need for agents which have dual H.sub.1 and PAF receptor antagonistic activity for the improved treatment of conditions mediated by histamine and PAF release. Such conditions include allergic rhinitis, sinusitis, asthma, dermatitis, psoriasis, urticaria, anaphylactic shock, conjunctivitis, pruritis, inflammatory bowel disease and colitis.
European patent specification EP-B-404797 (G. D. Searle) claims a series of PAF receptor antagonists of general formula (I) ##STR2## wherein Y, R.sup.1 and R.sup.2 are as defined in the publication itself.
International patent applications WO-A-92/03423 (British Biotechnology) and WO-A-9516687 and U.S. Pat. No. 5,180,724 (British Biotechnology) disclose series of compounds, incorporating a 2-methylimidazo4,5-c!pyridine group which are potent antagonists of the PAF receptor.
U.S. Pat. No. 2,712,020 (Burroughs Wellcome) and European patent applications EP-A-085959 and EP-A-133534 (Wellcome Foundation) disclose compounds which are potent H.sub.1 receptor antagonists.
International patent applications WO-A-92/14734 (Pfizer), WO-A-92/00293 (Schering), WO-A-89/10363 (Schering), WO-A-93/20080 (Schering), WO-93/20063 (Schering), WO-A-93/23400 (Schering), WO-A-93/02081 (Schering), WO-A-94/08581 (Toray), European patent applications EP-A-515158 (Schering), EP-A-463873 (Sankyo), EP-A-549364 (Sankyo), EP-A-577957 (Uriach) and Japanese patent application published under no 4-226993 (Yoshitomi) all disclose compounds which possess both histamine (H.sub.1) and PAF receptor antagonist activity.